The present invention relates to a semiconductor device and to an acceleration sensor and pressure sensor in particular for converting a displacement into an electrical signal by utilizing a piezo-resistance effect of a semiconductor crystal such as silicon.
With the recent development of micro-machining technology, it has become possible to form semiconductor acceleration sensors on a semiconductor substrate by forming and etching a thin film. An acceleration sensor fabricated typically by micro-machining is described in L. M. ROYLANCE, J. B. Angell: IEEE Transactions on Electron Devices, Vol. ED-26, No. 12, December 1979, for example. Micro-machining allows to thin a thickness of a diffused resistor section for chemically detecting a displacement to facilitate it to displace. According to Japanese Patent Laid-Open No. Hei. 1-302167, it is said that a diffused resistor section has been thinned by means of isotropic etching to enhance its sensitivity. FIG. 6 shows a prior art example of a semiconductor acceleration sensor. This prior art example requires a protection film 21 for protecting the surface wiring to thin the diffused resistance section by means of the isotropic etching.
Further, in order to prevent a destruction of the thin section, stoppers 1008 and 1009 for restricting a deformation shown in FIG. 70 have been used.
Next, a prior art technology of a semiconductor pressure sensor will be explained. Hitherto, there has been known a semiconductor pressure sensor in which a base 705 made from glass on which a base 702 made from silicon is bonded and a terminal 707 are adhered on a support base made from ceramics and the base 702 is connected with the terminal 707 by a wire 709 as shown in FIG. 34. Such a structure has been disclosed in "Electronics" 29, 6(1984) 17 for example. A pressure reference chamber 703 is provided and a diaphragm 704 is formed thereon also in such a prior art example.
Further, a micro-pressure sensor having a square diaphragm 704 having a side of 80 .mu.m has been made in trial by applying the micro-machining technology. This pressure sensor is characterized in that the diaphragm 704 and the pressure reference chamber are formed by etching from the surface of the silicon substrate 702. FIG. 35 shows a diagrammatic drawing showing a sectional structure of the pressure sensor fabricated by the micro-machining. Silicon of (100) face is used for the substrate and the diaphragm 704 is coated by a Si.sub.3 N.sub.4 film 713. Etching holes are bored around the diaphragm 704 and the right under the diaphragm is hollowed out to form a cavity. The etching holes are then sealed by the Si.sub.3 N.sub.4 film 713. In a case of the acceleration sensor, stoppers are used to prevent an excessive displacement as a measure for enhancing a shock resistance.
FIG. 60 shows a section view of the prior art semiconductor acceleration sensor. In FIG. 60, the semiconductor acceleration sensor comprises a cantilever 8102 formed by means of etching and a weight 8103 which is a mass section on a silicon base 8101, and diffused resistors for converting an acceleration into an electrical signal are formed on the cantilever 8102. An upper stopper 8105 and a lower stopper 8106 for preventing a destruction of the cantilever which may be caused when an excessive acceleration is applied are structured on and below the silicon base 8101.
When an acceleration is added to the acceleration sensor from a direction shown in the figure, i.e. from the top to the bottom of the figure, the cantilever 8102 deflects and resistances value of the diffused resistors change by the stress. The acceleration sensor detects the acceleration by taking out that variation.
However, the prior art semiconductor acceleration sensor of the semiconductor device fabricated by the micro-machining technology and the method of enhancing the sensitivity by thinning the diffused resistor section by means of etching have had the following problems:
1) A process for protecting other wiring and the like is required in order to perform etching to thin a displacement detecting section; PA1 2) A manufacturing process is prolonged due to a lithographic process such as an application of resist; PA1 3) An etching time and a thickness of the displacement detecting section vary considerably depending on a condition of an etchant because of a wet type process; PA1 4) Its shock resistance is weak because the diffused resistor section is thinned; and PA1 5) It takes time to fabricate the element because the diffused resistor section is thinned. PA1 1) Because the photolithographic process is employed, the manufacturing process such as an application of resist is prolonged; PA1 2) Because a strong base or the like is used in etching the diaphragm and the chemical is influential to parts other than the diaphragm, the process which may be used in the fabrication of the parts other than the diaphragm is limited; PA1 3) A time for processing the diaphragm and a finished thickness vary considerably depending on a condition of the etchant due to the wet-type process, thereby lowering a yield thereof; and PA1 4) Because the wet-type process is employed, a time for processing the diaphragm and a finished thickness vary considerably depending on a condition of the etchant, thereby lowering a yield thereof. PA1 1) Because the stopper for preventing an excessive displacement caused by a great acceleration hits with a distal end of the cantilever, causing an excessive displacement, an impact is exerted on the cantilever; PA1 2) A load is applied to the thin section having the diffused resistance; and PA1 3) Although a measure of filling silicon oil within a case of the acceleration sensor has been taken to reduce the impact, the installation of a manufacturing facility to fill the silicon oil is expensive. PA1 1) have a structure which allows to reduce the impact; PA1 2) reduce the load applied on the thin section having the diffused resistance; and PA1 3) reduce the manufacturing cost and enhance the shock resistance thereof. PA1 1) By constructing so that the diffused resistor side faces to the supporting substrate, the sensor element contacts with the wiring board when it displaces toward the diffused resistor side, thus preventing the destruction of the element; and PA1 2) By facing the wiring of the acceleration sensor to the wiring board, the wiring can be connected and outputs can be made readily.
The present invention allows a large number of acceleration sensor elements to be obtained from a semiconductor substrate and thereby to supply low cost acceleration sensors by a simple method of forming cantilevers by removing the substrate having diffused resistors.
Furthermore, the present invention allows a structure to be obtained having a strong shock resistance because the cantilever is constructed as a plane having a uniform thickness and no stress concentrates at any one part thereof.
While the method of thinning by the wet-type process is used when the pressure reference chamber is provided to detect a displacement by the diaphragm in the case of the pressure sensor of a semiconductor device, it has had the following problems similarly to the acceleration sensor:
Accordingly, it is an object of the present invention is to solve the aforementioned problems of the past by providing a semiconductor pressure sensor whose diffused resistors may be readily processed and which has a high sensitivity.
Next, a conventional destruction preventing mechanism will be explained with reference to FIGS. 61 through 63. In FIGS. 61 through 63, a base 801 is interposed between an upper stopper 803 and a lower stopper 804 and a deformation of the base 801 is limited for an excessive acceleration. FIG. 62 shows positions of the base 801, the upper stopper 803 and the lower stopper 804 in a state wherein no acceleration is applied. In states wherein an acceleration is applied in FIGS. 61 and 63, FIG. 61 shows a case when a destruction of the base 801 is prevented by the upper stopper 803. However, a load is applied to a thin section and there has been a possibility that the base 801 is destroyed from the thin section. In FIG. 63, there has been a possibility that the base 801 is destroyed by the lower stopper 804.
Accordingly, the destruction preventing mechanism of the prior art acceleration sensor has had the following problems:
Accordingly, further objects of the present invention are to: